The project uses a thin snake muscle to study the determinants of synaptic strength, or efficacy, at the neuromuscular junction. The project's focus is to examine the structure and function of synapses within individual motor units which have been identified in a living preparation. The first goal is to determine the quantal release parameters of individual boutons which comprise nerve terminals in snake muscles. Quantal content will be compared to bouton size and ultrastructure to determine if the terminal bouton serves an an anatomical correlate of synaptic strength. Second, synaptic strength will be compared to indicators of target cell requirements, such as input resistance, at all synapses supplied by one motor axon. This will permit the influence of target cell properties on synaptic strength to be independently assessed. Finally, synaptic strength will be systematically compared among all motor units within the muscle. This will permit the influence of motor neuron size and type to be independently determined for synapses whose target-related properties are precisely known. These measurements will determine the extent to which synaptic efficacy is an intrinsic motor neuron property, and how efficacy relates to other neuron properties. Knowledge of these relationships is essential in understanding how the size and strength of synapses are regulated, and why such regulation fails in diseases which attack pre- or postsynaptic cells.